This invention relates to a semi-reflective film and a reflective film for an optical information recording medium which is provided with a high cohesion resistance, high light resistance, and high heat resistance simultaneously with a high reflectivity, high transmissivity, low absorptivity, and high thermal conductivity in the field of optical information recording medium such as CD (Compact Disc), DVD (Digital Versatile Disc), Blu-ray Disc, and HD DVD; a sputtering target for an optical information recording medium which is used in depositing such a semi-reflective film or reflective film; and an optical information recording medium provided with such a semi-reflective film or reflective film.
Optical information recording mediums (optical discs) include various types, and the three main types categorized by the writing/reading system are read only, write-once, and rewritable optical discs. In order to increase storage capacity, single side, multilayer optical discs have also been developed from the ordinary single side, single layer optical discs. For example, in the case of a single side, dual layer optical disc wherein the signals are written and read in the recording layer which is farther from the side of the laser beam incidence, the laser beam should be transmitted through the recording layer which is nearer to the laser beam incidence, reflected by the farther recording layer, and again transmitted through the recording layer which is nearer to the laser beam incidence. Accordingly, a semi-reflective film capable of reflecting and transmitting the laser beam is used for the recording layer which is nearer to the laser beam incidence.
The materials which function as a semi-reflective film include metals such as Ag, Al, Au, Pt, Rh, and Cr and elemental semiconductors such as Si and Ge. Among these, pure Ag and Ag alloys containing Ag as their main component are featured in view of (1) high light efficiency (=reflectivity+transmissivity), (2) high reflectivity for the blue-violet laser (wavelength, 405 nm) used in Blu-ray disc and HD DVD, and (3) high thermal conductivity which enables adequate diffusion of the heat generated in the recording film upon recording of the signals. Such Ag based materials exhibit excellent properties for use as a semi-reflective film of an optical disc including high reflectivity, high transmissivity, low absorptivity (absorptivity=100%−(reflectivity+transmissivity)), and high thermal conductivity. However, in order to accomplish sufficient function as a semi-reflective film of an optical disc with long term reliability, the Ag based materials need to overcome the drawbacks of the Ag based materials, namely, (1) cohesion resistance, (2) light resistance, and (3) heat resistance.
[Drawback 1] Cohesion Resistance
Ag based materials are likely undergo cohesion under the effect of heat and halogen (fluorine, chlorine, bromine, iodine, etc.). When it is kept under the high temperature, high humidity conditions used in the reliability test of an optical disc, or disposed in contact with a halogen-containing organic material (of the organic dye recording film, the protective layer, or the adhesive layer), cohesion may take place, resulting in the increase in the surface roughness of the thin film or loss of the continuity of the thin film, which may in turn invite loss of the function of the material as a semi-reflective film or the reflective film.
[Drawback 2] Light Resistance
For example, a single side, dual layer, read only optical disc has a basic cross sectional structure of polycarbonate (PC) substrate/semi-reflective film/adhesive layer/reflective film/PC substrate. When such an optical disc is irradiated by a Xe lamp (a lamp having a spectrum resembling that of the sunlight) in the so called “light resistance test”, the semi-reflective film experiences decrease in the reflectivity when the film comprises a Ag based material, and in such a case, reading of the signal becomes impossible once the reflectivity is reduced to below the threshold value that is required for detecting the signal to be read.
[Drawback 3] Heat Resistance
For example, a single side, dual layer, write-once optical disc has a basic cross sectional structure of PC substrate/recording film/semi-reflective film/spacer/recording film/reflective film/PC substrate, and a single side, dual layer, rewritable optical disc has a basic cross sectional structure of PC substrate/dielectric and protective layer/interface layer/recording film/interface layer/dielectric and protective layer/semi-reflective film/intermediate layer/dielectric and protective layer/interface layer/recording film/interface layer/dielectric and protective layer/reflective film/PC substrate. In the case of recordable optical disc including such write-once and rewritable optical discs, the recording layer is heated to a temperature as high as 300 to 600° C. during the writing, and a very severe thermal hysteresis is applied to the semi-reflective film or the reflective film. The growth of the crystal grains of the thin film and loss of the continuity of the thin film caused as a result of such thermal hysteresis impair the function of the semi-reflective film and the reflective film.
Various attempts have been made to improve the pure Ag, mainly by alloying the Ag. For example, corrosion resistance is improved in U.S. Pat. No. 6,007,889 and the like by adding Au, Pd, Cu, Rh, Ru, Os, Ir, or Pt to Ag; in U.S. Pat. No. 6,280,811, Published Japanese translation of PCT international publication for patent application 2002-518596, and the like, by adding Au, Pd, Cu, Rh, Ru, Os, Ir, Be, or Pt to Ag; and in U.S. Pat. No. 5,948,497, Japanese Patent Application Laid-Open No. Heisei 6-208732, and the like by adding Pd or Cu to Ag. The inventors of the present invention have also disclosed in Japanese Patent No. 3365762 a method for improving crystal structure stability by adding Nd to Ag wherein the crystal structure stability is attained by suppressing Ag diffusion and crystal grain development.
In spite of such efforts, no Ag based alloy has been found that exhibits high reflectivity, high transmissivity, low absorptivity, and high thermal conductivity simultaneously with high cohesion resistance, high light resistance, and high heat resistance, and accordingly, there is a strong demand for a Ag based alloy which fulfill requirements for all these properties.